In the 504 major races this fall - for 33 Senate, 435 House, and 36 governor seats - the Democrats have basically secured their current share of seats. The softness - the margin for losses - has emerged mostly in Republican ranks.

That's not the way it was a year ago. Last November the GOP boasted they could put up a strong fight against 15 Democratic incumbent senators, and might win in at least half the races. The number dropped to 13 in February, and 12 as recently as July. At midsummer, the Republicans saw themselves vulnerable in just one Senate race.

Now the Republicans claim only three Democratic seats are within striking range - they trail in two of these races, and are even in one. And Republicans concede that at least four of their own Senate seats are vulnerable to Democrats. The Democrats see as many as seven GOP seats within their range.

In only two Democratic Senate seats have incumbents slipped since mid-September - Sen. Howard Cannon against Republican state Sen. Chic Hecht in Nevada, and Sen. Howard Metzenbaum giving a little ground to state Sen. Paul Pfeiffer in Ohio. Meanwhile, a number of Republican Senate candidates have lost ground - Sen. William V. Roth Jr. in Delaware, Rep. Millicent Fenwick in New Jersey, and Sen. David Durenberger in Minnesota.

If the close races split evenly, the outcome might look pretty much like a standoff. The Democrats might gain two seats - there's a 20 percent chance, they estimate, of winning four. If the latter happens, the victories would be enough to split the new Senate down the middle in party voting strength. Given the fact that the Democrats currently hold 20 of the 33 Senate seats being contested in 1982 (the Republicans hold the remaining 13), this represents a remarkable deflation of Republican prospects.

The situation is the same in the House races. A Congressional Quarterly (CQ) review of all the House contests puts the Democrats ahead in 239 races, two seats shy of the number they now hold. CQ rates 33 races even, and the Republicans ahead in 163 races. Consequently, Republicans are either even or slightly ahead in 196 House races. That about equals their current strength in the House of 192 seats. Even if the Democrats win a majority of the close races, the GOP would only lose from 15 to 20 seats. Still, that would be enough to give Democrats a working majority in the House.

In the governorships, too, the Democrats have already largely secured their current share of the races at stake and are in striking range of picking up 4 to 6 Republican seats. This would give the Democrats a far stronger base for 1984.

The difference in the Democrats' prospects in 1982 compared with 1980 is due largely to the state of the economy.

But in other respects Democrats have mostly gained ground as well. In California, Gov. Edmund G. (Jerry) Brown Jr. has offset his own high negative rating by drawing rival Pete Wilson - helped by Wilson's own mistakes - out into the open.

Democratic Sens. Paul S. Sarbanes in Maryland, George J. Mitchell in Maine, Daniel Patrick Moynihan in New York, Henry M. Jackson in Washington, Robert C. Byrd in West Virginia, Jim Sasser in Tennessee, and Quentin N. Burdick in North Dakota have been moving out of GOP target range. And suddenly it seems Republicans who had a clear edge - such as John C. Danforth in Missouri and Senator Durenberger - are on the defensive.